1. Field of the Invention
The present invention relates generally to control of an exposure when a color original is reproduced on a photosensitive material by a color photographic print enlarger, and more particularly to method of and a device for reproducing a color image with suitable tones.
2. Description of the Related Art
A known method of reproducing a color image, in which an image on a color original is projected onto a photosensitive material such as color photographic printing paper or a color film by a photographic print enlarger to expose the photosensitive material, proceeds as follows.
Filters of three primary colors, of subtractive color mixture, type, i.e., cyan (C), magenta (M) and yellow (Y), are inserted in sequence into an optical path through which a color original is projected onto a photosensitive material, so that exposure by each colored light is sequentially carried out. Controlling the amount of each filter inserted into the optical path adjusts the density of each filter to a desired value, thereby controlling tones of a reproduced color image.
The "density" of a filter represents the capacity of the filter to absorb light. A value of density is obtained by taking the logarithm of the measured ratio of the amount of incident light on the filter to the amount of light transmitted through the filter, or the ratio of the amount of light transmitted through a reference filter to the amount of light transmitted through an object filter. The logarithm of the ratio is taken due to the characteristic logarithmic response of the human to light. That is, the eye recognizes an increase in the actual amount of light logarithmically.
A cyan filter absorbs red to different degrees. Therefore, controlling the density of the cyan filter makes it possible to adjust the amount of red included in the exposure light. If a cyan filter having a larger density value is inserted into the optical path, for example, a smaller amount of red is included in the transmitted light. If a cyan filter having a smaller density value is employed, a larger amount of red is included in the transmitted light.
A magenta filter selectively absorbs green. A yellow filter selectively absorbs blue. Therefore, adjusting the density of the magenta filter and that of the yellow filter enables adjusting the amount of green and blue in the transmitted light.
As is well known, red, green and blue are the three primary colors of additive color mixtures. Changing the proportion of these colors makes it possible to obtain different tones. Therefore, a color image which is well balanced in color is obtained by properly adjusting, i.e. balancing, the respective densities of the cyan, magenta and yellow filters.
(1) A device provided in a color photographic print enlarger for tone control is described below.
The color photographic print enlarger has the characteristics described below.
(i) In the photographic print enlarger, Y, M and C color filters are provided between a color original and a light source, such that each of which filters can be inserted by a required amount into the optical path. Insertion of each filter by a required amount into the optical path varies the tones of light directed to the color original.
(ii) A projection lens is provided between the color original and a photosensitive material.
(iii) A photosensor for measuring the amount of light transmitted through the projection lens is provided between the projection lens and the photosensitive material.
(iv) Filters of the three primary colors, red (R), green (G) and blue (B), are provided on a photosensitive surface of the photosensor, each of which filters can be exchangeably provided on the photosensor.
(2) A conventional method of tone control in the color photographic print enlarger is described below.
(i) An original intended to be a reference for tone control (hereinafter referred to as the "reference original") is provided on the photographic print enlarger. Respective densities Dy.sub.0, Dm.sub.0 and Dc.sub.0 of the Y, M and C filters for reproducing the reference original with good tones are evaluated in advance.
(ii) The R filter is first provided on a photosensitive surface of a photosensor for measuring the amount of light, and a light source directs white light (light incorporating all visual wavelengths) to the R filter. The photosensor measures the amount of light transmitted through the reference original and the R filter. At this time, no Y, M and C filters are used on the side of the light source.
(iii) The amount of light transmitted through the other G and B filters are also measured, similarly to the R filter.
(iv) An original which is to be reproduced (hereinafter referred to as an "original to be processed" is substituted for the reference original to be provided onto the color photographic print enlarger.
(v) The amounts of light transmitted through each of the B, G and R filters are evaluated through the steps of (ii) and (iii).
(vi) The ratio of the amount of light of the respective colors B, G and R evaluated in the steps (ii) and (iii) to that evaluated in the step (v) is logarithmically converted. Respective values obtained by logarithmical conversion are added to Dy.sub.0, Dm.sub.0 and Dc.sub.0 (densities of the respective filters of Y, M and C for suitably reproducing the reference original). Thus, respective densities Dy.sub.1, Dm.sub.1 and Dc.sub.1 of the Y, M and C filters for reproducing the original to be processed with suitable tones are obtained. That is, the following expressions are given. ##EQU1##
In the above expressions, Dy.sub.1, Dm.sub.1, Dc.sub.1 represent respective filter densities to be set; B.sub.1, G.sub.1, R.sub.1 represent the measured amount of light transmitted through the original to be processed, the amount of which is logarithmically converted; and B.sub.0, G.sub.0, R.sub.0 represent the measured amount of light transmitted through the reference original, the amount of which is logarithmically converted.
(vii) The Y, M and C filters are inserted by desired amounts sequentially into the optical path so that effective filter densities may be Dy.sub.1, Dm.sub.1 and Dc.sub.1 as evaluated in the step (vi), whereby exposure by each colored light is carried out.
The exposure carried out through the above described procedures makes it possible to reproduce on the photosensitive material an original to be processed with different tones from those of the reference original, as a color image with suitable tones.
As mentioned in the foregoing, conventionally, the Y, M and C filter densities are corrected by correction values obtained in the expressions (1), based on the amount of light transmitted through the reference original and that transmitted through the original to be processed. However, there is a problem that the tones of the color image reproduced as described above do not always meet certain preferences. Moreover, the reproduced color image loses so-called "depth".
This degradation in image quality is comparatively small and thus rarely becomes a problem in amateur photography. Therefore, in general, densities of the Y, M and C filters are corrected by employing the correction values obtained by the above expressions (1). When an original is required to be reproduced with a higher quality of image, however, the degradation in image quality needs be eliminated as much as possible. In reproduction of an original to be used as a document to be applied to art printing, for example, the degradation in image quality described above must be eliminated.